The Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access Network (UTRAN) is a radio network of a UMTS system which is one of the third-generation (3G) mobile communication technologies, which provides circuit switched and packet switched services.
Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also referred to as LTE. Long Term Evolution is standardized by 3GPP Long Term Evolution (LTE) which is a project within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard with High Speed Packet Access functionality to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc.
An E-UTRAN typically comprises user equipments (UE) 100 wirelessly connected to radio base stations 200 a-d as illustrated in FIG. 1. In E-UTRA, Orthogonal Frequency Division Multiple (OFDM) is the access type that is used for the radio connection between the UE and the radio base station. Further, the radio base stations 200 a-d are directly connected to the core network (CN) 150 e.g. via an MME. In addition, the radio base stations 200 a-d are also connected to each other via an interface. The radio base stations are usually referred to as NodeB in UTRAN and to eNodeB in E-UTRAN.
The performance of packet based mobile networks relies on efficient channel-dependent scheduling. The network performs resource allocation or fast scheduling in the downlink by taking into consideration reported channel quality indicator (CQI) values from the UE. The CQI measurement is derived from the Signal to Interference and Noise Ratio (SIR) measured on a common reference or pilot symbols. An accurate channel quality indicator (CQI) at the eNodeB scheduler is crucial for successful operation of a packet based network such as LTE. It is important that the CQI is well-defined such that the scheduler has good knowledge of which transport formats a channel to a certain UE can support. For LTE, it has been decided that the CQI should correspond to a recommended transport format, presumably corresponding to some fixed block error rate (BLER) target.
In LTE, the CQI can be reported with several granularities, basically on a wideband or sub-band basis. If the CQI is reported on a wideband basis, one out of 16 CQI values is selected for the entire system bandwidth based on a measured SIR. Further, if the CQI is reported on a sub-band basis, one out of 16 different CQI values is selected for each sub-band. A sub-band is a pre-determined number of resource blocks, wherein a resource block is a physical resource on which data and control information can be transmitted. For example, a physical resource block may comprise a number of frequencies used during a limited time period. It should however be noted that it is up to the network/eNodeB to configure the CQI parameters, i.e., whether only wideband CQI should be reported or if a CQI down to the level of one or a few resource blocks should be reported.
Transmission parameters may be changed adaptively in order to adapt the transmission to the current interference situation, referred to as link adaptation. The link adaptation is performed based on e.g. reported and measured CQI. As mentioned above, the CQI reports comprise an indication of the signal to noise plus the interference ratio of the reference symbols over the frequency range. In the downlink (DL), the UE measures on pre-determined reference signals and reports CQI and power measurements, and provides information on how the channel quality differs over the frequency band. In the uplink (UL), the radio base station measures the quality of the received signal. If the UE also recently has used other parts of the frequency band, a certain awareness of the frequency dependency may be available to the radio base station. The CQI reports and the received signal level reports are then used to determine a suitable modulation and coding scheme (MCS), which is referred to as link adaptation. A MCS comprises coding scheme, bit rate, modulation, MIMO (Multiple Input Multiple Output) setting etc. The link adaptation is done separately per UE, and repeated for every n:th sub frame, which means that the scheduler needs to do link adaptation on all UEs. (In LTE networks the subframe is 1 ms.)
The SIR is estimated over a certain number of resource blocks (RBs). Then the SIR is mapped to a value for the CQI, for the same number of resource blocks as the estimated SIR. Given the actual bandwidth the CQI is reported for, the proposed MCS is in turn translated to a transport block size (TBS) based on the MCS and number of information data bits to be transmitted. However, the BLER (block error rate) depends on the actual TBS as will be described briefly in the following.
In LTE networks turbo codes are used for user data carried by the PDSCH and it is known that the error correcting performance of turbo codes depends on the size of the block that is processed. This is due to the “gain” introduced by the interleaver, which in a classical turbo coding setup with two parallel concatenated recursive systematic encoders separated by an interleaver as shown in FIG. 2 grows with increasing block size.
Hence, the BLER at a certain SIR for a specific channel is varying with the size of the transport block. Accordingly, small packets experience a different behavior in terms of BLER than large packets (e.g., “best-effort data” as in background-download services) for a given SIR.
Accordingly, there is a need for a solution taking the different behaviors of transport block with different sizes into account.